1. Field of the Invention
This invention relates to a mask for evaluation of an aligner used in a manufacturing process of a semiconductor device, and a method of evaluating an aligner using the same, and more particularly to an evaluation mask or an evaluation method preferably applicable for evaluation of an optical system of an aligner.
2. Description of the Prior Art
In the process for manufacturing a semiconductor device such as IC or LSI, etc., a lithographic process is an important process for forming various circuit patterns on a semiconductor substrate, or forming mask patterns used for ion implantation or diffusion.
FIG. 1 is a schematic cross sectional view of an aligner for lithography called a stepper (stepping projection aligner). Such a stepper is popularly used at present, and serves to repeatedly step a wafer with respect to a projection image of a pattern of a glass mask called a reticle to carry out exposure process. In this stepper, there is used a reticle serving as a glass mask 11 on which a pattern magnified so that its size is about five times larger than that of a pattern to be formed is provided as a chromium pattern, etc. Ultraviolet rays 12 from a lamp 15 are condensed by using a condensing unit 16 and focuses by using a focusing lens 17 to allow the rays thus focused to be incident to a reduction lens unit 13 through the reticle 11 to reduce them by the reduction lens unit 13 to project a mask image onto a photosensitive material such as photo resist coated on a substrate such as a semiconductor substrate, etc. And by exposing the rays on a photo resist (hereinafter simply referred to as a resist) film while stepping the substrate with respect to the projected mask image, a resist mask pattern is thereby formed.
Since the accuracy of a resist mask pattern formed by such lithographic process affects the accuracy of all patterns of a semiconductor device manufactured, the accuracy of the aligner is required to be severely controlled.
Particularly, since recent semiconductor devices are required to have larger capacity or higher integration, miniaturization of patterns and an enlargement in the exposure area, i.e., the chip size have been developed, so the accuracy required for an aligner or a lens therefore has become more severe. Accordingly, it is required that a finer pattern can be processed with good accuracy, and its processing accuracy is uniform in a broader exposure area.
In order to satisfy such requirements for the aligner, with respect to an aligner introduced into respective manufacturing lines, evaluation of lens accuracy, etc. is carried out at the time of introduction thereof and regularly. As the evaluation method, there is ordinarily used an evaluation pattern in which multi-directional standard calibration patterns are arranged at predetermined positions such as the central portion or the peripheral portion, etc. in an exposure area to allow those patterns to be exposed to light by various exposure times or focuses to compare those dimensions to carry out evaluation. At present, for the dimensional measurement, a high resolution Scanning Electron Microscopy (high resolution SEM) for measurement is used. In addition, in order to carry out control of dimensions of products actually manufactured, there has been also carried out a method of monitoring the dimension of a predetermined pattern of a product subject to exposure process at all times by measurement by the high resolution SEM to provide feedback of changes in the dimension upon occasion to adjust the exposure condition.
However, the dimensional measurement by the high resolution SEM has, in the first place, the drawback that a time required for one measurement is relatively long by focus adjustment of the high resolution SEM, etc. In addition, in order to carry out dimensional measurement with good accuracy, if an attempt is made to completely carry out positional alignment with respect to a pattern to be subjected to dimensional measurement, a manual adjustment by a person who carries out measurement is required. As a result, it is difficult to carry out automatic measurement. Accordingly, there is a limitation in an increase in the measurement point or the measurement period. Particularly, in evaluating curvature or aberration, etc. of a lens, dimensional comparisons at positions as many as possible within an exposure area are required. In accordance with measurement by the high resolution SEM, however, sufficient measurement is unable to be carried out when throughput of the line is taken into consideration. Further, with respect to patterns used in products, patterns at respective positions within an exposure area are generally different from each other. As a result, it is difficult to monitor changes in the processing dimension by curvature or aberration, etc. of a lens.
In addition, it is important for carrying out dimensional management to carry out not only lens evaluation, i. e., evaluation within an exposure area, but also obtain gross data of dimensions of wafer in-plane or respective lots to monitor changes in the deviation thereof. In the case of measurement by the high resolution SEM, because of restriction of throughput, etc., it is impossible to obtain gross data which can tolerate statistical evaluation such as deviation of measured values.